The need to secure websites and access to accounts causes a major headache for webmasters. Multiple techniques are used to help figure out who is human, but one of the most prevalent is the Captcha test.

The common Captcha presents a number of characters a user must type in to prove they are human. Those characters are not very clear, either by being written in some blurry text or hidden within a pattern. The idea being a human can still figure out what the characters are, while a computer has a hard time deciphering them.

Ever more sophisticated bots are able to decipher some of the most common plug-in Captcha tests, but those wanting to setup malicious accounts have also started turning to using cheap labor to fill out Captchas for them.

Most Captchas come with an audio option allowing you to hear the characters or words spoken in case you really can’t see them. Now it seems that audio option is a weakness as researchers have figured out how to interpret them as an automated process.

The system has been called Decaptcha and is the work of researchers at the Stanford and Tulane universities. It’s success rate depends on the Captcha being listened to. Here’s the percentage success rate for some of the most popular Captcha tests on the Web:

89% – Authorize.com

82% – eBay

48.9% – Microsoft

45.5% – Yahoo!

42% – Digg

1.5% – reCaptcha

Although the success rate for most audio tests is less than 50%, that doesn’t mean this technique isn’t effective. This is an automated system after all, and can just keep trying until it gets one right. Looking at it that way, 40% or over is good enough, and the researchers say a trained system can get 10 successes every minute.

Decaptcha only requires 20 minutes of training where it listens to 300 audio readouts from a specific Captcha system. It is then capable of picking out the characters by comparing them to a library it has built up.

Although the existence of Decaptcha may worry those who rely on Captcha systems for security, the fact it does exist actually helps improve audio Captchas. The researchers can now work with online services to improve the audio so it is much harder to decipher by Decaptcha.

Stanford and Telune aren’t the first to see some success with deciphering audio Captchas. Carnegie Melon built a system back in 2008 that saw a high rate of success too.